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ISSN : 1225-6692(Print)
ISSN : 2287-4518(Online)
Journal of the Korean earth science society Vol.39 No.1 pp.23-45
DOI : https://doi.org/10.5467/JKESS.2018.39.1.23

Trace Fossils from the Late Pleistocene Marginal Marine Deposits of Jeju Island, Korea: Implications for the Psilonichnus and Skolithos Ichnofacies

Jeong Yul Kim1*, Ji Hyun Kang2
1Department of Earth Science Education, Korea National University of Education, Cheongju 28173, Korea
2Jeju Science High School, Jeju 63144, Korea
Corresponding author: kimjy@knue.ac.kr+82-10-5498-1118+82-43-232-7176
20171129 20180130 20180211

Abstract

Moderately diverse, but very abundant trace fossils are found from the Late Pleistocene deposits of Jeju Island, Korea. Vertical I-, Y- and U-shaped domichnia of annelids or decapods are, over 2500, extremely abundant, 3D network domichnia of callianassids are, over 200, very abundant, and small sinuous trails of nematode repichnia are, over 50, abundant in number. Horizontal trails attributable to polychaete or worm-like animals are, less than 50, common, but horizontal spreiten burrows, fish traces and crab trackways are, less than 10, rare in occurrence. Of these trace fossils, Taenidium barretti, Undichna britannica and Undichna unisulca represent the first record from the Pleistocene in Asia. Psilonichnus upsilon is the second record in Asia. Crab trackways probably produced by underwater punting gait of sideway walking crabs may represent the first record in the world. In addition, diverse and very abundant footprints of more than 500 hominids, more than 200 birds and more than 1000 mammals are closely associated with these invertebrate trace fossils. Trace fossil assemblage integrated with sedimentary facies is interpreted to have been formed in the marginal marine foreshore to backshore environment corresponding to the Psilonichnus and Skolithos ichnofacies.


초록


    Introduction

    General environmental significance of trace fossils was noted by Seilacher (1964, 1967), who has recognized several distinct environmental groupings of trace fossils named “ichnofacies”. Like facies models of diverse sedimentary environments, trace fossil facies model including several ichnofacies has been also developed (Frey and Pemberton, 1984; Pemberton et al.,1992a, b). The Ichnofacies Paradigm has evolved across a nearly sixty –decade period and ichnofacies provides a logical first step in the interpretation of depositional settings (MacEachern et al., 2007, 2012).

    The marginal marine environments represent a dynamic transitional setting composed of marine, quasimarine, and nonmarine (terrestrial) conditions. Trace fossil assemblages are particularly useful when studying marginal marine paleoenvironments, given the high variations in sedimentary parameters and complex facies organization found in these depositional settings (Fernández and Pazos, 2015). Seilacherian marginal marine ichnofacies includes the Psilonichnus and Skolithos ichnofacies. The Psilonichnus ichnofacies (e.g., Frey and Pemberton, 1987) represents moderateto low-energy supralittoral to upper littoral marginal marine environment and commonly consists of vertical J- , Y- and U- shaped dwelling, crawling and foraging traces with vertebrate tracks. The Skolithos ichnofacies (e.g., Seilacher, 1967; Howard and Frey, 1984; Droser, 1991) represents moderate to high-energy lower littoral to sublittoral setting and mainly comprises vertical, cylindrical, branching, and (or) U-shaped dwelling burrows, and equilibrium and escape traces.

    Skolithos, one of typical ichnogenus of the Skolithos ichnofacies, has been frequently recorded from the deep sea (Ekdale, 1977) to nonmarine (Netto, 2007) deposits since the Ediacaran (Crimes, 1994). However, trace fossils belonging to the Psilonichnus ichnofacies or ichnocoenose have been rarely reported except for those from the Jurassic of Scotland (Marshall et al., 2002), Eocene of Japan (Myint, 2008), Holocene and Pleistocene of the Georgia coast (Frey and Pemberton, 1987), and modern seaward side of Peixe Lagoon (Brazil) (Netto and Grangeiro, 2009). Furthermore, tracks and trackways of diverse birds and mammals as well as many invertebrate trace fossils have rarely documented in ancient marginal marine deposits corresponding to the Psilonichnus and Skolithos ichnofacies, though they are potentially diagnostic and preservable (Frey and Pemberton, 1987).

    The fossil site named as “the Hominid and Other Vertebrate Tracksite of Namjeju” was discovered in 2000 and contains over 500 hominid footprints and diverse and numerous avian and mammalian tracks as well as many invertebrate trace fossils. Of these, hominid footprints and vertebrate tracks have received many attention (Kim et al., 2008a, b, 2009; Kim and Lockley, in preparation). However, little attention was given to extremely abundant invertebrate trace fossils preserved in the fossil site.

    The purpose of this paper is to describe moderately diverse, but very abundant invertebrate trace fossils, which are co-occurred with diverse and numerous avian and mammalian tracks, and to interpret depositional environment on the basis of distribution of trace fossils integrated with sedimentary facies from the Late Pleistocene deposits of Jeju Island, Korea. The Jeju trace fossils composed of moderately diverse, but extremely abundant invertebrate and vertebrate traces represent a unique and remarkable example for more understanding the Late Pleistocene subaquous and subaerial paleoecology and paleoenvironment corresponding to the Psilonichnus and Skolithos ichnofacies.

    Geological Setting and Sedimentary Facies

    The Jeju Island of Korea is mainly composed of Quaternary volcanic rocks and a few Pleistocene to Holocene sedimentary deposits such as the Seogwipo, Hamori, and Sinyangri formations. In the southwestern area of the Jeju Island, where the fossil site is located, the Pleistocene Kwanghaeak Basalt is widely distributed (Fig. 1). In the near west of Mt. Song-ak, the unnamed strata unconformably overly the Kwanghaeak Basalt and it is overlain by the Song-aksan Tuff, which is covered by sand dune. However, in the northeast of Mt. Song-ak, the footprint-bearing strata, which were shown as the Hamori Formation (Park et al., 2000), overly the Kwanghaek Basalt and they were overlain by sand dune (Kim and Kim, 2006; Kim et al., 2009).

    In the measured section of the footprint-bearing strata with about 2 m in thickness, four sedimentary facies are recognized (Fig. 2). They are the laminated mudstone/shale facies (Ml), mudstone-fine-grained sandstone couplet facies (M-Sf), laminated fine- to medium-grained sandstone facies (Slfm), and crossstratified coarse-grained sandstone facies (Sxc).

    The Ml facies consists of laminae of gray to light gray mudstone or shale. Most laminae are very- thinly laminated. This facies shows good lateral continuity and symmetrical wave ripple marks are commonly preserved. Desiccation cracks are uncommonly observed in a few bedding surface. Invertebrate trace fossils in this facies are Cochlichnus anguineus, Protovirgularia dichotoma, and crab trackways. Abundant and diverse morphotypes of bird and artiodactyl tracks are also recognized in this facies. On the basis of lithology, sedimentary structures, invertebrate and vertebrate trace fossils, and body fossils, this facies is interpreted to have been formed in very shallow subaquous and rarely subaerial lower foreshore environment.

    The M-Sf facies is composed of alternating layers of thinly laminated (1 to 2 mm thick) dark gray to gray mudstone (or shale) and light gray fine-grained sandstone. This facies shows good lateral continuity and it is about 5 to 15 cm in thickness. Symmetrical wave ripple marks are commonly preserved on the mudstone surface in this facies. Hominid footprints, artiodactyl tracks, and diverse bird tracks, Arenicolites ichnosp., Skolithos linearis, Thalassinoides suevicus, and crab trackways are preserved in this facies. On the basis of lithology, ripple marks, diverse and abundant vertebrate tracks and invertebrate trace fossils, and crab fossils (Fig. 3), this facies is interpreted to have been formed in the lower to upper foreshore environment.

    The Slfm facies consists of laminated fine- to medium-grained light gray sandstone. This facies is laterally continuous and it is about 3 to 25 cm in thickness. Symmetrical ripple marks and cross bedding are infrequently preserved. Vertebrate tracks including hominids, carnivores, artiodactyls, proboscideans and birds, and fish trails including Undichna unisulca and U. britannica are found in this facies. Vertical burrows (Skolithos linearis, Arenicolites ichnosp. and Psilonichnus upsilon), 3D network burrows (Thalassinoides suevicus), and horizontal traces (Palaeophycus tubularis, Taenidium barretti, T. satannassi and crab trackways) are preserved in this facies. Small gastropod and bivalve, and plants are uncommonly observed in this facies. This facies is interpreted to have been formed in upper foreshore environment.

    The Sxc facies comprises brown colored very coarse-grained sandstone, which is uncommonly intercalated with granule- to pebble- bearing conglomerate in some places. This facies is laterally continuous and it is up to 65 cm in thickness. The Granule- to pebble- bearing conglomerate is interpreted to have been formed by strom- induced erosion of underlying basalt. Cross bedding and normal graded bedding are commonly preserved, and desiccation cracks and ripple marks are uncommonly observed in this facies. Psilonichnus upsilon and artiodactyl tracks are sporadically found in this facies. This facies is interpreted to have been formed in the backshore environment.

    In summary, on the basis of lithofacies and vertebrate and invertebrate ichnofossils, the hominid footprint-bearing strata of the Late Pleistocene deposits in Jeju Island have been formed in subaquous shallow marine upper shoreface and subaerial backshore environments, where diverse birds, mammals including hominids, and moderate but very abundant invertebrates frequently visited or habitually lived.

    Systematic Ichnology of Invertebrate Trace Fossils

    Ichnogenus ArenicolitesSalter, 1857

    Type ichnospecies: Arenicola carbonaria Binney, 1852 by subsequent designation (Richter, 1924).

    Diagnosis: Vertical U-tubes without a spreite (after Fürsich, 1974).

    Discussion: Arenicolites differs from Diplocraterion Torell, 1870 in the absence of a spreite (Hakes, 1976). It is considered to be the dwelling and feeding burrow of suspension-feeding (particularly polychaete) annelids (Hakes, 1976; Chamberlain, 1978a) or crustacean-like organisms (Goldring, 1962). Though typically a shallowwater marine form, Arenicolites has been also reported from nonmarine (Bromley and Asgaard, 1979) and deep-water deposits (e.g., Crimes et al., 1977, 1981). Arenocolites ranges in geological age from the earliest Cambrian Phycodes pedum Zone of Narbonne et al. (1987) to the Holocene (Chamberlain, 1978b).

    Arenicolites ichnosp. Fig. 4-A, B

    Material: Several thousand specimens in the field. Collected specimen JITF 15001.

    Description: Specimens occur as vertical U-plane sections and paired circular openings on bedding surfaces of coarse-grained sandstone with about 5 cm thick. Burrow height up to 20-28 mm, width 9-13 mm. Tubes are nearly vertical, parallel, of equal diameter (1.9-2.6 mm) and are typically sandstonefilled with a thin (less than 0.5 mm) mud lining. UFig plane sections show an evenly rounded base. Orientation of paired openings is not clearly recognized due to densely scattered distribution on the bedding surface.

    Remarks: Without observation of U-plane sections, specimens appear to resemble single opening of Skolithos. Specimens differ from Arenicolites curvatus, A. compressus, and A. subcompressus in having a nearly circular cross-section. U-plane sections are much smaller and too regular to represent A. variabilis but do resemble A. carbonaria. Specimens are also different from A. stather; and A. naraensis in having only a thin wall-lining.

    Ichnogenus CochlichnusHitchcock, 1858

    Type ichnospecies: Cochlichnus anguineusHitchcock, 1858 by monotypy

    Diagnosis: Regularly meandering, horizontal trails and burrows resembling sine curve (after Hitchcock, 1858; Häntzschel, 1975).

    Remarks: Cochlichnus is eurybathic and is of Late Proterozoic to Holocene age (Fedonkin, 1988; Metz, 1987). Its occurrence in the upper Vendian Redkino Horizon of the Russian Platform (Fedonkin, 1988) indicates that Cochlichnus rarely underlies the uppermost Precambrian Harlaniella podolica Zone of Narbonne et al. (1987). Although it has been known as a facies indicator (e.g., the Upper Carboniferous cyclothems of West Germany (Hakes, 1976), Cochlichnus has been reported from submarine-canyon (Pickerill, 1981) and flysch (Ksia¿kiewicz, 1977) successions, delta-slope sediments (Eagar et al., 1985), tidal flats (Narbonne, 1984; Hiscott, 1982), supersaline epeiric seas (Courel et al., 1979), river flood plains (Fordyce, 1980), point bars (Archer and Maples, 1984), proglacial lakes (Gibbard and Stuart, 1974; Gibbard, 1977) and bayous (Chamberlain, 1975).

    Cochlichnus has been referred to annelids lacking well-developed parapodia (Hitchcock, 1858; Hakes, 1976). Moussa (1969, 1970) reported that nematodes, which lack circular muscles and thus move by flexing the body in the dorsoventral plane (Clark, 1964), make sinusoidal trails similar to Cochlichnus, where mud is covered by a film of water not thicker than their body. Insect larvae can also produce similar trails under such conditions (Metz, 1987).

    Cochlichnus anguineusHitchcock, 1858 Fig. 4-C, D

    Material: Several hundred specimens in the field. Figured specimen JITF 15002.

    Diagnosis: Smooth Cochlichnus.

    Description: Very small, smooth, unbranched, sinusoidal trails preserved in convex hyporelief on thin mudstone beds. Diameter is 0.3-0.4 mm and length up to 50 mm. Amplitude of sine curve is 1.8-2.4 mm and wave length is 2.9-3.4 mm.

    Remarks: Specimens are quite similar in size and shape to C. anguineus described from the Oligocene Freshwater Molasse of Switzerland (Uchman et al., 2004). ichnospecies of Cochlichnus, such as C. kochi, C. serpens, are regarded to be junior synonym of C. anguineus (Fillion and Pickerill, 1990). C. supruliformis Yang and Hu in Yang et al. 1987 is probably nonbiogenic (Fillion and Pickerill, 1990). Keighley and Pickerill (1997) reviewed the status of nine ichnospecies of Cochlichnus and concluded that only C. annulatus and potentially C. antarcticus were morphologically distinct from the type ichnospecies, C. anguines. Uchman et al. (2004) provided synonymy list of C. anguineus and divided into tight forms and stretched forms of C. anguineus on the basis of wave length/amplitude ratio.

    Ichnogenus Helminthopsis Heer, 1877

    Type ichnospecies: Helminthopsis magna Heer, by subsequent designation (Ulrich, 1904).

    Diagnosis: Unbranched, irregularly winding or meandering, interface burrows or trails that do not touch, cross, or loop themselves. A maximum of one order of nonsinusoidal meandering is present. Burrow fill unstructured (after Han and Pickerill, 1995).

    Discussion: Han and Pickerill (1995), Wetzel and Bromley (1996), Wetzel et al. (1998), and Pickerill et al. (1998) have reviewed this ichnogenus, rationalizing the twenty-two previously described ichnospecies into a more manageable three ichnospecies. Wetzel and Bromley’s (1996) criteria for diagnosing ichnospecies are either highly arbitrary or poorly defined. Accordingly, following Han and Pickerill (1995) and Pickerill et al. (1998), the three valid ichnospecies are H. abeli Ksi뗌 kiewicz and H. hieroglyphica Heer, that are distinguished by differences in the surficial winding geometry (see the ichnospecific diagnosis below), and H. granulata Ksiažkiewicz that regardless of surficial geometry, is recognized by a marginal pelletal ornamentation.

    Helminthopsis has been known from the lowermost Cambrian strata of the Phycodes pedum Zone (Narbonne et al., 1987; Narbonne and Myrow, 1988) to the Pleistocene (Wetzel, 1981). It is an eurybathic from, though more frequently reported from deepwater flysch successions (Pickerill, 1981), and is generally regarded as having been produced by polychaete annelids and possibly priapulids.

    Helminthopsis hieroglyphica Heer in Maillard, 1887 Fig. 4-D

    Material: One specimen JITF 15002-1.

    Description: One distinct specimen (center in Fig. 4- D) is a uniformly thin (about 1.5mm wide), smooth, compressed, irregularly meandering, interface trail or burrow with straight sections. Other adjacent, loosely winding burrows on the same bedding surface may represent additional examples of this ichnospecies. The specimen is preserved as a convex hypichnion on the base of a mudstone containing Cochlichnus anguineus.

    Remarks: Assignment of this specimen to Helminthopsis, as opposed to the distinctly sinusoidal Cochlichnus, is based upon the structure ‘curving back’ on itself in a partial horseshoe-or C-shaped pattern. Such doubling back is not element for assignment to Cochlichnus. The irregularly meandering burrows with straight sections direct the ichnospecific assignment to H. abeli.

    Ichnogenus PalaeophycusHall, 1847

    Type ichnospecies: Palaeophycus tubularisHall, 1847 by subsequent designation (Miller, 1889, p. 130).

    Diagnosis: Straight to slightly curved to slightly undulose or flexuous, smooth or ornamented, typically lined, essentially cylindrical, predominantly horizontal structures interpreted as originally open burrows; borrow-fill typically massive, similar to host rock; where present, bifurcation is not systematic, nor does it result in swelling at the sites of branching (Fillion and Pickerill, 1990).

    Remarks: Palaeophycus is distinguished from Planolites by having a wall and the same fill as the host rock (Pemberton and Frey, 1982). Five ichnospecies have been recognized by these authors: P. tubularisHall, 1847, P. striatusHall, 1852, P. heberti (Saporta, 1872), P. sulcatus (Miller and Dyer, 1878), and P. alternatusPemberton and Frey, 1982. Palaeophycus is interpreted as a dwelling structure (domichnion) of a suspension feeder or predaceous organism (Pemberton and Frey, 1982). The predaceous polychaete Glicera has been suggested as a modern analogue of the Palaeophycus tracemaker (Pemberton and Frey, 1982). Systematic review of Palaeophycus was made by Keighley and Pickerill (1995), and Buckman (1995), who erected P. crenulatus for all passively filled burrows characterized by welldeveloped annulated lining. Palaeophycus is an eurybathic facies-crossing form, probably produced by polychaete, and it has been reported from the Precambrian to Holocene (Pemberton and Frey, 1982).

    Palaeophycus tubularisHall, 1847 Fig. 4-E

    Material: About 30 specimens in a bedding surface; figured slab (JITF 15003).

    Description: The specimens are preserved in concave epirelief on very thin mudstone covered coarse-grained sandstone. The burrows are unbranched, slightly curved, horizontal to slightly inclined, cylindrical and commonly collapsed. Burrows is about 7.5 mm in diameter and up to 80 mm in length. Wall-lining is distinct and thin, mostly about 0.5 mm. Burrow-fill is massive and similar in lithology to the enclosing host rock. Some burrows are incompletely filled and collapsed. Burrow surface is smooth and does not display any ornamentation.

    Remarks: Palaeophycus tubularis is distinguished from P. heberti by the consistently thicker wall lining of the latter, and from other ichnospeceies of Palaeophycus with thin-lining by the absence of persistence, well developed striae and annuli (Pemberton and Frey, 1982).

    Ichnogenus PlanolitesNicholson, 1873

    Type ichnospecies: Planolites vulgarisNicholson and Hinde, 1874, by subsequent designation (Miller, 1889).

    Diagnosis: Essentially cylindrical, predominatly (sub)horizontal but bedding-penetrative, straight to tortuous, ornamented of smooth, unlined burrows. Unbranched or non systematically branched, lacking swelling at any ramification points (modified from Fillion and Pickeril, 1990).

    Remarks: Pemberton and Frey (1982) discussed in detail on the Planolites-Palaeophycus dilemma and systemically reviewed the two common and simple ichnogenera. They recognized only three distinct forms, namely Planolites beverleyensis (Billings), P. montanus Richer, and P. annulatis Walcott among the ichnospecies of Planolites currently known in the literature. However, criteria suggested by them for distinguishing P. beverleyensis from P. montanus was not clear, rather ambiguous. Keighly and Pickerill (1997) considered, therefore, the latter ichnospecies as a subjective junior synonym of the former, as previously suggested by Clausen and Vilhjalmsson (1986). Ichnotaxonomy of this ichnogenus has been documented in detail by Fillion and Pickerill (1990) and Keighley and Pickerill (1997).

    Planolites is an eurybathic, extremely faciescrossing form and it has been reported from the Precambrian to Holocene (Häntzschel, 1975).

    Planolites montanusRichter, 1937 Fig. 4-F

    Material: About 10 specimens observed in a slab (JITF 15004)

    Description: Nearly straight, slightly curved, unlined, horizontal to slightly inclined burrows preserved in concave epirelief on the sandstone with ripple marks. Trough of ripple marks is partly covered with thin mudstone. Burrow-fill is finer-grained than surrounding host rock. Wall lining and branching is rarely observed. Burrows are less than 3.7 mm in diameter and observed up to about 60 cm in length. Diameter of burrows commonly varies probably due to preservation of burrows on the sandstone bed with ripple marks partly covered with thin mudstone in the trough parts.

    Remarks: The specimens show all the attributes to Planolites monatanus, to which they are confidently assigned. P. montanus is distinguished from the other ichnospecies of Ichnogenus Planolites by its relatively small size (diameter) with curved to contorted course (Pemberton and Frey, 1982).

    Ichnogenus ProtovirgulariaMcCoy, 1850

    Type ichnospecies: Protovirgularia dichotomaMcCoy, 1850, by monotypy.

    Diagnosis: Small, plaited, unbranched, keel-like trail, mostly straight or slightly curved, more rarely sinuous, generally paired and bilaterally symmetrical, narrow wedge-shaped appendages (after Han and Pickerill, 1994).

    Remarks: The ichnogenus Protovirgularia is a relatively commonly reported form, particularly from the Southern Upland of Scotland, from where McCoy (1850) described the type material regarded as an octocoral. Seilacher and Seilacher (1994) erected a new ichnofamily Pelecypodichna, to which the ichnospecies of Protovirgularia as well as Lockeia James belongs and they recognized four ichnospecies of Protovirgularia, namely P. dichotoma, P. tuberculata (Williamson), P. longespicata (Stefani), and P. rugosa (Miller and Dyer). They regarded the ichnogenera including Crossopodia M’Coy, Walcottia Miller and Dyer, Pennatulites Stefani, Paleosceptron Stefani, Biformites Link, Uchrites Macsotay, Imbrichnus Hallam, and Sustergichnus Chamberlain, as junior synonyms of Protovirgularia (Seilacher and Seilacher, 1994). The nomenclatural history and detailed taxonomic considerations of Protovirgularia have been discussed by Seilacher and Seilacher (1994), Han and Pickerill (1994), and Uchman (1998), who also described P. dichnotoma, P. pennatus, P. rugosa, P. obliterata, P. longespicata, P. vagans, P. dzulynskii, and P. isp from the flysch deposits of Polish Carpathians.

    Provirgularia dichotomaMcCoy, 1850 Fig. 4-G

    Material: One specimen (JITF 15005).

    Description: The specimen is well preserved in concave epirelif on the surface of fine-grained grey sandstone. It is unbranched, straight to gently curved, horizontal, and keel-like trail with maximum width of 27 mm and length of 16.5 cm. V-angles (appendage amples with respect to the median line, Han and Pickerill, 1994) measured are more than 130°. Lateral appendages are about 1.3 mm in width and up to 7.5 mm in length, and dichotomous and nearly symmetrical with respect to the prominent median line. Continuous median line is about 2 mm in width and 1.5 mm in height. Twenty appendages are observed in each side of the median line. Six small tridactyl bird tracks are preserved near the specimen on a single bedding surface.

    Remarks: Protovirgularia dichotoma was established by McCoy (1850) for structures that possessed a median line and associated lateral appendages. Ichnotaxonomy of P. dichotoma was documented in detail by Han and Pickerill (1994) and emended diagnosis of this ichnospecies was provided by Uchman (1998).

    Ichnogenus Psilonichnus Frsich, 1981

    Type ichnospecies: Psilonichnus tubiformis Frsich, 1981.

    Diagnosis: Predominantly vertical, cylindrical, unlined burrows ranging from irregular shafts to crudely J-, Y- , or U- shaped structures (Frey et al., 1984).

    Remarks: Frey et al. (1984) provided taxonomic key for identification of ichnogenera most likely to include crabs as tracemakers. They grouped the ichnogenera into the unbranched burrows (Skolithos and Macanopsis), sparsely branched burrows (Gyrolithes, Spongeliomorpha, and Psilonichnus), and well-integrated burrow systems (Thalassinoides). Psilonichnus is distinguished in J-, Y-, or U- shaped, erect components from the other sparsely branched burrows (Frey et al., 1984).

    Psilonichnus has been proved to be extremely useful indicators of marginal marine environments and the past sea-level positions (Frey et al., 1984; Frey and Pemberton, 1987; Gingras et al., 2000; Marshall et al., 2002; Nesbitt and Campbell, 2006).Psilonichnus has been reported from the Upper Jurassic to Pleistocene marginal marine deposits (Fürsich, 1981; Gingras et al., 2000).

    Psilonichnus upsilonFrey et al., 1984 Fig. 4-H

    Material: Over two hundred specimens in the field; figured specimen (JITF 15006).

    Diagnosis: Psilonichnians consisting typically of gently inclined, sparsely branched to unbranched, J- or Y- shaped burrow; inclined shafts straight to slightly arcuate; branches slightly to markedly curved, not horizontal (Frey et al., 1984).

    Description: Unbranched to Y- shaped, unlined burrows; shafts, steeply inclined to bedding, typically 5 mm in diameter and up to 4 cm or more in length. Where branched, angle of bifurcation is about 50°-60°. Terminus of burrows not preserved; therefore, measured shafts length are less than the original length of shafts.

    Remarks: Up to now, four named ichnospecies of Psilonichnus have been currently known. They are P. tubiformis, P. upsilon, P. lutimuratus, and P. quietus, which most likely is a junior synonym of P. tubiformis (Nesbitt and Campbell, 2006). Burrows from the Pleistocene of Jeju Island are identified as Psilonichnus upsilon on the basis of burrow morphology, which corresponds to diagnostic features of the ichnospecies provided by Frey et al. (1984). Psilonichnus upsilon is distinguished in inclined shapes-bearing relatively few, or no, branches from P. tubiformis (Fürsich, 1981) and in unlined burrows from P. lutimuratus (Nesbitt and Campbell, 2002). Jeju specimens represent the second record of Psilonichnus upsilon in Asia.

    Ichnogenus SkolithosHaldeman, 1840

    Type ichnospecies: Fucoides? linearisHaldeman, 1840 by monotypy.

    Diagnosis: Unbranched, vertical or steeply inclined, cylindrical or subcylindrical, lined or unlined burrows. Walls distinct or indistinct, smooth or rough, possibly annulate; burrow fill massive; burrow diameter may vary slightly along its length (after Alpert, 1974).

    Remarks: Alpert (1974) reviewed systematically the ichnogenus Skolithos and he recognized five valid ichnospecies, namely, Skolithos annulatus (Howell), S. ingens Howell, S. linearis, S. magnus Howell, and S. verticalis (Hall). Although his scheme is regarded by many ichnologists as adequate, some taxonomic problems such as overlapping nature of diameter ranges of S. linearis, S. magnus, and S. verticalis still remain to be solved. Anderson and Sauvagnat (1998) described a new ichnospecies, S. grandis for a very large, cylindrical, erect tubes with typical diameter approximately 5 cm, and with length as much as 6 m or more from the Permian of western USA. Skolithos has been reported from deep marine to nonmarine, Late Precambrian to Holocene deposits (Fillion and Pickerill, 1990). Gregory et al. (2006) interestingly documented plant traces resembling Skolithos from the Quaternary and Holocene deposits of New Zealand and Georgia, USA.

    Skolithos linearisHaldeman, 1840 Fig. 5-A, B

    Material: Several thousand specimens in the field, figured specimen JITF 15007.

    Description: Vertical to slightly inclined, straight, cylindrical burrows. Burrows are 3 to 7 mm in diameter and up to 18 cm in length. Burrow walllining is distinct and very thin (less than 0.2 mm thick). On the horizontal bedding surface, browncolored concentric features with maximum diameter up to 50 mm, around one to five openings of burrow are distinctly observed. The concentric features are also vertically developed around the vertical burrows. Burrow-fill is coarse-grained sandstone, which is clearly different from the lithology of the surrounding host rocks of mudstone. Annulation is commonly observed on the outer part of vertical burrows observed in vertical section.

    Remarks: According to Alpert (1974) who provided systematic review of Skolithos, specimens are assigned to be Skolithos linearis. Burrows without showing downward or upward branching and changing in burrow diameter are not likely to be plant traces resembling Skolithos, which is recently documented in Quaternary and recent deposits of New Zealand and George of USA (Gregory et al, 2006). Specimens are also different from plant trace resembling Skolithos in lacking hematite-cemented conical walls up to 10 or 15 cm high.

    Brown colored, concentric, vertical cylinder-like features developed around vertical burrow of Skolithos linearis are characteristic. They are regarded to be not wall lining but diagenetic oxidation haloes because there is no alteration in sediment fabric (Magwood, 1992). These diagenetic haloes are interpreted to be probably due to oxidation around vertical burrows by meteoric or ground water resulting these peculiar concentrically color-banded structures.

    Ichnogenus Taenidium Heer, 1877

    Type ichnospecies: T. serpentinum Heer, by subsequent designation (Häntzschel, 1962).

    Diagnosis: Unwalled, essentially cylindrical, meniscate, backfilled burrows. Variably oriented in a straight, winding, curved, or sinuous pattern. Secondary successive branching may occur, but true branching is absent (after Keighley and Pickerill, 1997).

    Remarks: Ichnotaxonomy of simple, unbranched meniscate burrows such as Ancorichnus, Beaconites, and Taenidium has been reviewed by D’Aessandro and Bromley (1987) and Keighley and Pickerill (1994). Ancorichnus is a burrow containing a central meniscate fill and a structured mantle (Heinberg, 1974), Beaconites is a walled meniscate burrow (Keighley and Pickerill, 1994), and Taenidium is an unwalled meniscate backfilled burrow (Keighley and Pickerill 1994).

    Taenidium barrettiBradshaw, 1981 Fig. 5-C

    Material: Three specimens (JITF 15008).

    Description: Horizontal to slightly inclined, unbranched, unwalled, meniscate backfilled burrows preserved in light gray mudstone. Menisci are hemispherical, tightly packed. Meniscate fill is heterogeneous. Menisci are about 1 mm. Diameter of burrow slightly varies due to slight inclination. Burrows are 7 to 12 mm in diameter and length of burrows is up to 125 mm.

    Remarks: Taenidium barretti is distinguished from other ichnospecies of Taenidium in hemispherical or deeply arcuate, tightly packed meniscate backfill (Keighley and Pickerill, 1994). Taenidium barretti has been recorded from alluvial lacustrine and fluvial deposits of the Ordovician to Pleistocene (Keighley and Pickerill, 1994). Jeju specimens represent the first record of Taenidium barretti from the marine shoreline deposits and the first Pleistocene record in Asia.

    Taenidium satanassi D’Alessando and Bromley, 1987 Fig. 5-D

    Material: Two specimens contained in a slab (JITF 15009).

    Description: Horizontal, gently curved to straight, unbranched, meniscate backfilled burrows preserved in concave epirelief on light gray mudstone. Specimens are 6.6-8.0 mm in diameter and up to 80 mm in length. Menisci are weakly arcuate. Sediment packets are about 4 to 5 mm in length, resulting considerably shorter than wide. Several specimens of Skolithos occur with Taenidium satanassi on a single bedding surface.

    Remarks: Typically a slight constricting annulation in the outer boundary of the burrow corresponds to the contact between the packets (Keighley and Pickerill, 1994). Taenidium satanassi is distinguished from T. barretti in lacking hemispherical or deeply arcuate and tightly packed meniscate backfill, and from T. cameronensis and T. serpentinum in meniscate packets, which are considerably shorter than wide (Keighley and Pickerill, 1994). T. satanassi has been rarely known and only described from the Permian to Miocene deposits of China, India, and Italy (D’Alessando and Bromley, 1987; Hu et al., 2010; Tiwari et al., 2011). Therefore, Jeju specimens represent the youngest record of T. satanassi and the first record from the Pleistocene deposits in the world.

    Ichnogenus ThalassinoidesEhrenberg, 1944

    Type ichnospecies. Thalassinoides callianassae Ehrenberg, by original designation (=Thalassinoides suevicus?, Kennedy, 1967).

    Diagnosis: Large burrow systems consisting of smooth-walled, essentially cylindrical components; branches are Y- to T-shaped, typically enlarged at points of bifurcation; burrow dimensions may vary within a given system (after Howard and Frey, 1984).

    Remarks: Thalassinoides is a facies-crossing trace fossil, most typical of shallow-marine environment, and is produced mainly by crustaceans (e.g., Frey et al., 1984). Origin and palaeoenvironmental significance of Thalassinoides were lately summarized by Ekdale (1992). Thalassinoides occurs profusely in Ordovician marine limestones all over the world (Archer and Maples, 1984; Sheehan and Schiefelbein, 1984; Ekdale, 1992). Thalassinoides ichnofabric representing the firm-substrate related to transgressive surfaces is useful in sequence stratigraphy (Sharafi et al., 2012).

    Thalassinoides suevicus Rieth 1932 Fig. 5-E

    Material: Several hundred specimens in the field; figured specimens (JITF 15010).

    Description: Predominantly horizontal, more or less regularly branched, smooth surfaced, cylindrical large burrow systems observed on bedding surface of mudstone. Y-shaped dichotomous branching is commonly preserved. Enlarged swelling is observed at branching parts of burrows. Thin lining of burrow is preserved. Diameter of burrow is up to about 25 mm. Inclined shafts uncommonly observed are relatively short, less than 15 cm. Burrow fill is different in color and lithology from the surrounding host rock.

    Remarks: Thalassinoides suevicus is distinguished form T. paradoxicus by the highly variable, irregularly branched burrows commonly with T-shaped intersections (Howard and Frey, 1984).

    Ichnogenus Undichna Anderson, 1976

    Type ichnospecies: Undichna simplicitasAnderson, 1976.

    Diagnosis: Trace fossils comprising a single horizontal wave, or set of horizontal waves of common wavelength and direction of travel. Waves may be paired and parallel, or unpaired. Unpaired waves may be intertwined or separate. The waves may be continuous or just their troughs/crests preserved. Waves may also be superimposed upon a straight, continuous furrow/ ridge. A pair of straight, parallel grooves/ ridges may also accompany the wave (after Minter and Braddy, 2006).

    Remarks: Anderson (1976) first described fish trails as ichnogenus Undichna from the Early Permian of South Africa. Subsequently many reports about Undichna have been documented (e.g., Higgs, 1988; Trewin, 2000; Gibert et al., 1999; Simon et al., 2003).

    Minter and Braddy (2006) provided emended diagnosis of Undichna and ichnospecies of Undichna, and summary of stratigraphic, geographic and environmental distributions of Undichna. Nine valid ichnospecies of Undichna have been described from the Devonian to Pleistocene deposits of all continents except Antarctic (Minter and Braddy, 2006). In addition, Undichna was more recently reported from the Triassic of Italy by Todesco and Avanzini (2008), and Permian of Antarctica (Jackson et al., 2016).

    Undichna britannica Higgs, 1988 Fig. 5-F

    Material: One specimen (JFTF 15001).

    Description: Trails comprising two out-of-phase, interwined waves, preserved in concave epirelief on mudstone. Wave length of sinusoidal trails is about 70 mm and amplitude is up to about 9 mm. Trails are slightly asymmetrical. Length of trail observed is 140 mm. Width of trail is thin, about 1.4 mm.

    Remarks: Undichna britannica has been recorded from the Devonian to Pleistocene deposits of diverse environments (Minter and Braddy, 2006, Table 1). Higgs (1988) attributed it to the anal fin. Fish trails described as Undichna britannica have been reported from the Triassic of Shaanxi Province, China (Lu and Chen, 1998; Wang et al., 2008). The Jeju specimen represents the first Undichna britannica recorded in Korea and the second record from the Pleistocene deposits in the world.

    Undichna unisulcaGibert et al., 1999 Fig. 5-G

    Material: One specimen (JFTF 15002).

    Description: Horizontal trail consisting a single sinusoidal wave. Additional lateral waves are absent. Wavelength is about 68 mm and amplitude is about 11 mm. The amplitude/wavelength ratio is about 0.16. Width of sinusoid groove varies from 4.5 to 9.0 mm; much wider in crest or trough parts. Continuous longitudinal striations are observed in some part of groove. Length of trail is up to about 183 mm. Trails are preserved as epichnial grooves on light grey mudstone overlying coarse-grained sandstone. Incomplete parts of curved trails adjacent to sinusoidal trail are observed.

    Remarks: Gibert et al. (1999) mentioned that regularly sinusoidal trails, Cochlichnus is distinguished from Undichna unisulca in having a much smaller amplitude and wavelength and in being less sharply incised. Undichna unisulca is interpreted to be produced by fish with a large caudal or anal fin (Gibert et al., 1999).

    It is regarded that change in width (especially wider in crest or trough parts) and longitudinal striations as well as smaller amplituds and wavelength may be additional criteria for distinguishing Undichna unisulca from regularly sinusoidal Cochlichnus anguineus. Fish trails described as Undichna unisulca was recorded from the Upper Pleistocene of Austria (Fliri et al., 1970, 1971). Recently, Undichna unisulca was described from the Lower Triassic of Sichuan Province, China (Lu et al., 2012). Undichna unisulca of Jeju Island represents the first record of this ichnospecies from the Pleistocene deposits in Asia and the second record from the Pleistocene deposits in the world.

    Crab trackway Fig. 5-H

    Material: Several trackways observed in the field; figured trackway (JCTW 15001).

    Description: Straight to slightly curved trackways forming with repeated sets composed of three seedlike, laterally and parallely placed, separated impressions of dactylus of crab preserved in convex epirelief or convex hyporelief of mudstone bed. Figured trackway is 86 mm in length and about 13 mm in width. Trackway consists of five sets (I to V). Distances between sets I and II, II and III, III and IV, and IV and V are about 23, 13, 18, and 19 mm, respectively. A set is composed of three impressions (outer, middle, and inner ones), which are distinguished in size and shape. The outer impression is slender seed-like in shape, and about 7.2 mm in length and about 1.6 mm in maximum width. The middle impression is more robust seed-like or elongated triangular in shape, and about 7.2mm in length and about 2.4mm in maximum width. The inner impression is very slender seed-like in shape, and about 8 mm in length and about 1 mm in maximum width. All impressions are sharply pointed to same direction (toward right). Opposite ends to pointed end of impressions are distinctively depressed down to bedding surface. Distance between outer and middle impressions is about 9 mm and that between inner and middle impressions is approximately 4 mm. Angles between lines connecting outer-middle impressions and those connecting inner-middle impressions measured in sets I to IV are about 75, 118, 132, 140, and 150°, respectively, resulting gradual increasing angles from set I to set V.

    Remarks: Hydrodynamics of sideway walking of crabs in the water has been documented (e.g., Martinez et al., 1998; Martinez, 2001). Martinez et al. (1998) reported that crab in water used a novel gait named ‘underwater punting’, characterized by alternating phases of generating thrust against the substratum and gliding through the water to prevent overturning and washing away in strong current. In this regard, crab trackways with repeated sets, which are composed of three impressions attributable to three dactylus of walking legs (first, second, and third legs) except large movable finger and fourth or swimming leg, are probably produced by underwater punting gait of sideway walking crabs.

    Crab fossils found in the tracksite of Jeju Island belong to Ocypodoidea, which is most likely producer of crab trackways. Several dozens of ichnogenus and several hundreds of ichnospecies for arthropod tracks or trackways have been reported (e.g., Seilacher, 1955; Häntzschel, 1975; Walter, 1983). In author’s knowledge, however, ichnotaxa resembling crab trackways of Jeju Island have rarely been described yet. Further study with more specimens may possibly allow to erect ichnotaxon for Jeju crab trackways.

    Trace Fossils, Ichnofacies, and Paleoenvironments

    As well as diverse and abundant avian and mammalian tracks, invertebrate trace fossils herein described are moderately diverse, but very abundant. As shown in Table 1, 14 ichnotaxa and crab trackways are described. They are Arenicolites ichnosp., Cochlichnus anguineus, Helminthopsis hierogryphica, Palaeophycus tubularis, Planolites bevereyensis, Protovirgularia dichotoma, Psilonichnus upsilon, Skolithos linearis, Taenidium barretti, Taenidium satanassi, Thalassinoides suevicus, Undichna britannica, Undichna unisulca, and crab trackways. Of these, Taenidium barretti, Undichna britannica, and Undichna unisulca represent the first their record from the Pleistocene deposits in Asia, and Taenidium satanassi represents the youngest and first its record from the Pleistocine in the world.

    In the study area, diverse and abundant avian and mammalian tracks as well as over 500 hominid tracks also have been described (Kim and Lockley, in preparation). Bird tracks described are Jejuornipes kangi, Ardeipeda koreanensis, Ardeipeda ichnosp., Aviadactyla jejuensis, and Avipeda sagaeriensis. Mammalian tracks described are Odocoileinichnium jejuensis, O. hallaensis, Bifidis circuliformis, Felipeda ichnosp., Canipeda ichnosp. A, Canipeda ichnosp. B, and Prodoscipeda enigmatica.

    Ethologically, I-, Y- and U-shaped vertical tubes of domichnia are extremely abundant (over 2500), and 3D network burrows of domichnia and sinuous repichnia are very abundant or abundant in number of invertebrate trace fossils. Horizontal tubular domichnia are common and other repichnia, pascichnia, and natichnia are very rare to rare in occurrence (Table 1).

    Possible producers for invertebrate trace fossils are annelids including polychaetes, callianassids, crabs, bivalves, and fishes (Table 1). Of these, trace fossils attributable to annelids are most abundant in number (over 2500). Trace fossils attributable to crustaceans are also considerably abundant in number (over 200). The paleocommunity is interpreted to be composed mainly of annelids and crustacea with minor amount of bivalves in shallow marine foreshore to backshore environment, where diverse and abundant shorebirds and mammals frequently visited for feeding and other purposes.

    Figure 6 is the schematic diagram showing paleoenvironmental distribution of lithology, physical sedimentary structures, body fossils, invertebrate trace fossils described in this study, vertebrate tracks, and corresponding ichnofacies. As described before, four lithofacies recognized on the basis of lithology including sedimentary structures integrated with trace fossils are interpreted to have been formed in the marginal marine foreshore to backshore environments.

    The Psilonichnus ichnofacies represents backshore to upper foreshore environments with a mixture of marine, quasimarine, and nonmarine conditions (Frey and Pemberton, 1987; MacEachern et al., 2007). The Psilonichnus ichnofacies slightly overlaps the Skolithos ichnofacies, which may correspond to lower foreshore and shoreface environments (Frey and Pemberton, 1987; MacEachern et al., 2007, 2012). The Psilonichnus ichnofacies is composed predominately of vertical, cylindrical I-, Y- or U-shaped burrows (Arenicolites, Psilonichnus, and Skolithos) with a few 3D network burrow systems (Thalassinoides) and horizontal meniscate burrows (Taenidium) (Fig. 6). Psilonichnus is a typical ichnogenus of the Psilonichnus facies. Generally it is well known that Psilonichnus is a significant ichnogenus indicating marginal marine environment (e.g., Frey et al, 1984; Frey and Pemberton, 1987; Nesbitt and Campbell, 2006; Wroblewski, 2008; Netto and Grangeiro, 2009).

    Occurrence of Psilonichnus, Skolithos, Arenicolites and Thalassinoides in the Psilonishnus ichnofacies was also reported from the Pleistocene Banas carbonates of western India (De, 2005). This ichnocoenose composed mainly of domichnia and fodinichnia is referable to moderate to relatively high energy marginal marine Psilonichnus ichnofacies. Extremely abundant vertebrate tracks including tracks of hominids, birds, and mammals are significant ichnofauna of the Psilonichnus ichnofacies. Most bird tracks are attributable to shorebirds and mammalian tracks are mostly of artiodactyls. Although Frey and Pemberton (1987) emphasized vertebrate lebensspuren in the Holocene Georgia coast corresponding to the Psilonichnus ichnofacies, occurrence of extremely abundant vertebrate tracks including over 500 hominid footprints associated with abundant invertebrate trace fossils has rarely been recorded in the marginal marine deposits referable to the Psilonichnus ichnofacies.

    The Skolithos ichnofacies is mainly composed of vertical, cylindrical, I-shaped burrows (Skolithos), Yshaped burrows (Psilonichnus), and U-shaped burrows (Arenicolites), 3D network burrows (Thalassinoides), and very small horizontal traces (Cochlichnus). Subordinate traces are Helminthopsis, Palaeophycus, Planolites, Protovirgularia, Taenidium, Undichna, and crab trackways (Fig. 6). The Skolithos Ichnofacies (Seilacher, 1967) is indicative of moderate to relatively high energy condition commonly found in nearshore terrigeneous clastic setting (Droser, 1991).

    Predominant vertical and cylindrical trace fossils such as Skolithos and Arenicolites, which are suspension feeding and dwelling traces, may indicate opportunistic animals colonizing during storm-like events (Pemberton and Frey, 1984; Pemberton et al., 1992c). Trace fossil assemblages forming the Skolithos ichnofacies indicate moderate to relatively high energy littoral environment. Nearly similar assemblages of trace fossils have been also recorded from the Skolithos ichnofacies (MacEachern et al., 2007; Gani et al., 2009; Rajkonwar et al., 2014)

    In addition, very abundant vertebrate tracks including hominid, bird, and mammalian footprints occur in the Skolithos ichnofacies. As mentioned before, Holocene coastal deposits corresponding to the Skolithos ichnofacies of Georgia barrier islands contains numerous and diverse shorebird and mammalian tracks (Frey and Pemberton, 1987). However, avian and mammalian fossil tracks have been rarely reported in the Skolithos ichnofacies, which has been frequently and worldwidely documented from the Cambrian to Holocene deposits. Occurrence of very abundant avian and mammalian tracks as well as characteristic invertebrate trace fossils in the Skolithos ichnofacies strongly indicates very shallow foreshore environment.

    Occurrence of trace fossils from the Pleistocene Jeju Island suggests that vertebrate tracks including shorebird and mammalian tracks are powerful indicator as marginal marine environments especially of shoreline settings. Furthermore, trace fossils of Jeju Island may represent an unique example supporting Frey and Pemberton (1987), who emphasized vertebrate lebensspuren as well as invertebrate trace fossils in the marginal marine environments.

    Acknowledgments

    Authors thank two anonymous reviewers for their useful comments and helpful reviews, which very helped for improvement of this manuscript. We also thank Hye Bin Shim, Graduate School of Korea National University of Education, who helped in drawing figures and word processing.

    Figure

    JKESS-39-23_F1.gif

    Geologic map showing fossil locality (modified form Park et al., 2000).

    JKESS-39-23_F2.gif

    Measured section showing stratigraphic distribution of lithology, sedimentary structures, body fossils, and trace fossils from the Late Pleistocene deposits of Jeju Island, Korea (modified from Kim et al., 2009; Kim and Lockley, in preparation).

    JKESS-39-23_F3.gif

    Outcrop view of fossil site (A), symmetrical wave ripple marks (B), hominid footprints (C), large bird tracks (D), replica of artyodactyl tracks (E), and crabs (F) from the Late Pleistocene deposits of Jeju Island, Korea. Scale bars in B and E are 8.6 cm in length, scale bar in C is 8 cm long, scale length in D is 40 cm, and coin in F is 2.2 cm in diameter.

    JKESS-39-23_F4.gif

    Trace fossils of Arenicolites ichnosp. (A, B), Cochlichnus anguineus (C, D), Helminthopsis hieroglyphica (D), Palaeophycus tubularis (E), Planolites montanus (F), Protovirgularia dichotoma (G), and Psilonichnus upsilon (H) from the Late Pleistocene deposits of Jeju Island, Korea. Coins in A-D, and F are 2.2 cm in diameter, camera cap in E is 7 cm in diameter, coin in G is 2.3 cm in diameter, and ballpoint pen in H is 1 cm in width.

    JKESS-39-23_F5.gif

    Trace fossils of Skolithos linearis (A, B), Taenidium barretti (C), Taenidium satanassi (D), Thalassinoides suevicus (E), Undichna britannica (F), Undichna unisulca (G), and crab trackway (H) from the Late Pleistocene deposits of Jeju Island, Korea. Camera caps in A, B, and E are 7 cm in diameter, coins in C and F are 2.3 cm, and coins in D, G and H are 2.2 cm in diameter.

    JKESS-39-23_F6.gif

    Schematic diagram showing paleoenvironments, geomorphology, and paleoenvironmental distribution of lithology, body fossils, invertebrate trace fossils, vertebrate tracks, and corresponding ichnofacies.

    Table

    Invertebrate ichnofaunas from the Late Pleistocene deposits of Jeju Island, Korea

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